Cultured mouse fibroblasts which are transformed by RNA viruses, a DNA virus or a chemical agent, all secrete a 35,000 Mr-glycoprotein (major excreted protein, MEP) in large amounts. Nontransformed murine fibroblasts secrete this protein in much lower amounts. These fibroblasts and cultured primary mouse epidermal cells, the target for tumor promoters in vivo, can be stimulated to release MEP by treatment with tumor promoters. We have purified this protein, prepared monospecific antisera against it, and begun to clone a cDNA which codes for MEP. The protein, of unknown biologic function, undergoes extensive modification prior to secretion. One of these modifications is the addition of mannose 6-phosphate, the lysosomal recognition marker. in non-transformed cells from which MEP is not secreted, MEP appears to have a predominantly lysosomal localization. We are studying this system as a model of regulation of protein synthesis, processing and secretion as it is affected by transformation and agents which mimic the transformed state, such as tumor promoters.